Distinguish temporary and permanent comments

## Problem
Comments in code may serve more than one purpose:
- Informative comments in addition to the code
- Code commented out for development purposes

It would be useful to distinguish the two so that the latter can be quickly found and removed from a final PR.

At present, `M` places `//` at the start of the text (indented if required) and so it is indistinguishable from an informative comment.

## Proposal
When toggling comment on code lines with `M` the line is commented out with (e.g. `//~`) at the very start of the line. Informative comments can then use `//` or `/*`. Moreover a search for `//~` will find the temporarily commented out code quickly.

## Prior Art
Geany uses `//~` at the start of the line for toggled comments.